The unit's newly installed. When you start it up after a few seconds you can hear something spinning up and then making a constant humming (not musical) or whirring sound as long as the unit is on. About 30 seconds after the unit turns off, the sound shuts down. Any thoughts come to mind?

Probably something touching the fan at the rear of the unit. You might want to have that looked at.

Thanks. I'll do that.

That was the solution. Thanks!

Glad I coud help.